CENOZOIC PLANKTONIC FORAMINIFERA DIAGENETICALLY ALTERED TO SIDERITE IN LOMONOSOV RIDGE SEDIMENTS, ARCTIC OCEAN

Cenozoic sediment recovered during drilling operations along the Lomonosov ridge in the central Artic by Integrated Ocean Drilling Program (IODP) Expedition 302 contain unique botryoidal grains of apparent authigenic origin. These grains first appear at 153.7 mcd in mid Miocene sediments and sporadically occur up-section throughout the drill cores in varying amounts. The relative abundance of the grains varies from 0-98% of each sieved (>250 um) sample's total volume and this percentage fluctuation can take place within as little as a 2.48 mcd depth range. Currently the stratigraphic occurrence of these grains is being compared to variations in sediment lithology and pore water chemistry. While these grains vary in color (tan to brown), abundance (0-98%) and individual size (250-500 um), they all encompass a distinct texture and shape which makes these grains easily distinguishable. They can be found as individual grains intermixed with the terrigenous sediments, but more commonly they adhere to one another in globular clusters. The source for these grains appears to be planktonic foraminifera which have settled from the overlying surface waters to the top of the Lomonosov ridge and gone through diagenetic mineral alteration. SEM analysis indicates that the original calcareous shells of the plankton have been altered into siderite, an Fe-carbonate with traces of Mn and Al, though some of the youngest grains found far up-section indicate alterations dominated by Mn, creating the Mn-carbonate, rhodochrosite. The diagenetic alteration has dramatically distorted the once smooth white shells into generally yellowish-brown nodules with rough texture. However, many grains clearly still resemble the foraminifera shape by showing 4 to 5 remnant chambers along with a residual aperture. The discovery of the siderite grains and the proposed formation by diagentic alteration of planktonic foraminifera would explain the lack of well preserved foraminifera observed by the Expedition Scientific Party during initial core inspection.